Modernize UI with glass morphism and refined styling

- Add backdrop-filter blur effects to sidebar, cards, and dropdowns
- Replace flat button colors with gradients
- Implement softer multi-layer shadows
- Add new CSS variables: --color-overlay, --color-card-outline, --color-focus-ring
- Modernize transitions with smoother easing curves
- Add loading spinner animation
- Add hexToRgbString() for --color-accent-rgb variable

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/styles/theme/dimensions.css

@@ -32,7 +32,7 @@
   --radius-sm: 4px;
   --radius-md: 6px;
   --radius-lg: 8px;
-  --radius-xl: 12px;
+  --radius-xl: 20px;
   --radius-full: 9999px;
 
   /* Component Heights */
@@ -151,4 +151,4 @@
   --breakpoint-mobile: 768px;
   --breakpoint-tablet: 1024px;
   --breakpoint-desktop: 1280px;
-}
+}